Brandon, a children's swimming instructor, is a liar and a bully who claims his pupils are all after him. And now he's been seen comforting a frightened boy with a kiss on the lips. But is any of that enough to justify the parents rounding on him with accusations of being a pervert?

In the local town, anxieties are already high following an unspecified incident at the youth centre, and Archimedes' Principle taps in to every parent's fears about the opportunistic predators just waiting to attack a child.

So playwright Josep Maria Miro Coromina, in a translation by Dustin Langan, asks us to consider whether our security-obsessed society has gone too far when a simple gesture of affection can lead to a cascade of terrifying recriminations.

Director Marta Noguera-Cuevas has drawn together an excellent cast, but the potential subtlety of this exploration is weakened by the characterisations in the writing itself. Brandon, played with swagger and aggressive assurance by Lee Knight, is such an unsympathetic character that the scales are weighed far too heavily against him.

He jokes about having sex with a mother in front of her "bitch" daughter. He's Facebook friends with a pupil, and boasts to his aghast colleague that he can already spot the gay boys.

Nor is he averse to using his own nakedness as an open threat to his boss, Anna, when she challenges him – behaviour that would surely justify dismissal regardless of anything else he may or may not have done.

Desperate to be fair, Anna opens her discussions with Brandon looking more like an anxious mum than an angry boss. The tragic reason for her being so keen to give him the benefit of the doubt is gradually revealed, and Kathryn Worth's performance is one of stirring emotional depth.

But once again, the play presents problems. It beggars belief that an experienced manager would tackle an issue of this gravity in an open changing room, with Brandon parading in his swimming trunks and other staff members at liberty to walk in.

Anna's appeasing weakness as a female boss, and Brandon's brazen observation that no one would want to impregnate a woman with such a "bitter expression", both suggest a sexism that's built into the play much more subtly than its more obvious theme.

Matt Bradley-Robinson is impressive as Matt, the good-guy instructor who sticks to his decent, principled approach to the job. And Julian Sims as David, the angry dad gunning for Brandon, gives a powerful account of parental fears.

Cory Roberts' set and Max Pappenheim's sound design create a convincingly claustrophobic atmosphere in the echoing locker room.

Archimedes' Principle sets out to challenge and disturb, and succeeds in doing both.